1. Field of the Invention
The present invention relates to a circuit for controlling the operation of a solenoid valve, and more particularly to a solenoid valve control circuit which employs a battery as a power supply.
2. Description of the Relevant Art
Some washroom faucets have an automatic water supply control unit for automatically supplying water by actuating a faucet solenoid valve when the approach of a user to the faucet is detected, and for automatically stopping the water supply by actuating the solenoid valve again when the leaving of the user from the faucet is detected.
Generally, such solenoid valve comprises a plunger serving as a valve body and a latching solenoid for driving the plunger when it is energized. As shown in FIG. 4A of the accompanying drawings, it is empirically known that the solenoid valve has a certain characteristic relationship between a power supply voltage Vcc applied to the solenoid and the total coulombs, or charge Q (i.e., all the electric current flowing through the solenoid, "coulombs" or "charge" being hereinafter referred to as an "electric quantity") through the solenoid. When the power supply voltage Vcc is low, the electric quantity Qn which is required by the solenoid to drive the plunger is larger than the electric quantity Qn that is required by the solenoid to drive the plunger when the voltage Vcc is sufficiently high. Stated otherwise, the electric quantity Qn which is required and sufficient to drive the plunger has to be passed through the solenoid for a relatively long time when the power supply voltage Vcc is lower and for a relatively short time when the power supply voltage Vcc is higher.
Where a battery is employed as the power supply for the solenoid valve and the solenoid is to be energized for a constant period of time, a problem arises either when the voltage Vcc of the battery is higher because the battery is new or when the voltage Vcc of the battery is lower because the battery is old or deteriorated. More specifically, if the time for which the solenoid is to be energized is selected to be relatively short in view of new battery conditions, then the solenoid will not be sufficiently energized when the battery voltage Vcc becomes lower and the plunger will not be driven to a desired stroke. Conversely, if the time of energization of the solenoid is selected to be relatively long in view of old or deteriorated battery conditions, then the solenoid will be excessively energized when the battery voltage Vcc becomes higher, resulting in excessive electric power consumption and a shorter battery service life.
The present invention has been made in view of the aforesaid problems with conventional solenoid valve control circuits.